Home testing is becoming an important market for diagnostic assays. Examples include home test kits for pregnancy, ovulation, & occult blood. It is common in such tests to provide a disposable device that has indicator reagents that react directly with the analyte of choice in the body fluid being tested, to produce a visual indication of the presence or absence of that analyte. As an example of the latter, immunoassays for infectious disease may require the subsequent addition of another liquid containing an appropriate label that will attach to the indicator layer and produce a detectable change, only if the analyte in question is significantly present in the body fluid.
Thus the disposable device is preferably both a storage container for at least some of the reagents involved, in dried form, and a reaction chamber to develop a visually observable change when the body fluid is added.
Examples of disposable devices that have been provided for such a use include those described in U.S. Pat. Nos. 3,825,410 and 3,888,629, issued on 7/23/74 and 6/10/75, respectively. Both of these feature a container with at least one compartment that has an upper portion and a lower portion. In the container of the '629 patent, the upper portion contains a stored reagent for reaction with the sample liquid, and a filter matrix. Preferably the reagent is stored on or in the matrix, which provides an indicator surface. The lower portion contains means for absorbing liquid from the upper portion through the filter matrix. The upper and lower portions are confined between side walls, and the absorbing means extends the full width of the side walls.
It has been found that the difficulty with such a device is that, because the absorbing means are in contact with the side walls at the place of contact with the filter matrix, the surface tension of the liquid at the side walls causes excessive amounts of solution to be drawn through the filter matrix during the absorbing step, at the walls. As a result, reaction product formed during the reaction is non-uniformly distributed, with a higher concentration at the walls.
One approach to this problem has been to fix the reagent to a prescribed central part of the filter matrix. In such a device, the reagent and the resulting reaction product cover an area having a symbolic shape, such as a plus sign or a minus sign. As a result, however, the reaction occurs only while the liquid flows through the filter matrix. That is, the reagent is not capable of floating free into free liquid above the matrix. However, if it were not so fixed to a central area, so that it could diffuse into liquid temporarily held above the matrix, thereby increasing the rate of reaction, there would be incurred the problem noted in the previous paragraph.
Therefore, prior to this invention there has been a need for a disposable container of the type described, which produces a more uniform signal at the indicator surface, without requiring that the reagent be somehow fixed just to a central area of the indicator surface.